The described invention relates to dural cutting devices that may be operated manually, using a single hand.
Dura Mater
The dura mater is the dense fibrous layer of tissue that covers the brain and spinal cord of mammals. As shown in FIG. 1 (Stedman's p. 548), the dura is comprised of an inner meningeal layer and an outer periosteal or endosteal layer. The layers comprising the dura are fused together in most places, but separate where necessary to accommodate meningeal vessels and large venous (dural) sinuses, and where septa between brain regions are formed. The periosteal layer attaches directly to the inner surface of the skull and provides vascular and fibrous extensions into the skull bone. At the large opening in the base of the skull, which forms the passage from cranial to spinal cavity, the periosteal layer is closely adherent with bone, and is continuous with the dura matter of the spine.
The cellular and extracellular composition of the cranial dura mater includes fibroblasts, large amounts of extracellular collagen, and some elastic fibers arranged as flat layers. These structures are imperfectly separated by blood vessels and lacunar spaces to form the meningeal layer and the periosteal layer.
The outer, periosteal layer is rough and composed of fibers (i.e. is fibrillated) that adhere to the inner surface of skull bone. The adhesion to the inner skull is most pronounced opposite the immovable joints between skull bones (i.e. cranial sutures). The periosteal layer contains the blood vessels that supply blood to skull bone.
The meningeal layer has, on its inner surface, a layer of highly specialized elongated fibroblast cells named dural border cells. This layer does not contain any collagen and the cells lack cell junction connections. The cells of this layer are often separated by extracellular space that comprises amorphous non-filamentous material. The meningeal layer itself comprises two more layers: the compact lamella and the loose lamella. The compact lamella comprises tight fibrous tissue with few blood vessels, while the loose lamella does contain blood vessels.
The dura mater also is involved in the formation of the various dural folds or septa. The meningeal layer of cranial dura mater sends inward four septa (dural folds) that divide the cranial cavity into freely communicating spaces lodging the subdivisions of the brain. These folds are: (1) the falx cerebri, (2) the tentorium cerebelli, (3) the falx cerebelli, and (4) the diaphragma sellae.
The falx cerebri is a sickle shaped fold of dura mater that lays in the midline between the two cerebral hemispheres. It descends vertically from the longitudinal fissure between the hemispheres. The falx cerebri is narrow in the front of the skull near the root of the nose (i.e. where it is attached to the ethmoid bone at the crista galli) and is broader at the rear of the skull where it connects with the upper surface of the tentorium cerebelli. Its upper margin is convex and attached to the inner surface of the skull in the middle line. This attachment begins as far back as the internal occipital protuberance. The upper margin of the falx cerebri contains the superior sagittal sinus. Its lower margin is free and concave, and contains the inferior sagittal sinus.
The tentorium cerebelli is a cresent shaped fold of the dura mater that forms a roof over the posterior cranial fossa. It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemisphers. The tentorium cerebelli bounds a large oval opening in the anterior, which is used for transmission of the large bundle of corticofugal nerve fibers that pass longitudinally over the ventral surface of the midbrain on each side of the midline. This opening also allows transmission of sensory and autonomic fibers and other fiber tracts. The tentorium cerebelli is attached to the transverse ridges on the occipital bone. Three sinuses are related to the tentorium cerebelli: the straight sinus, which runs along the attachment of the tentorium to the falx cerebelli; the superior petrosal sinus, which runs along the attachment of the tentorium to the petrous bone; and the transferse sinus, which runs along the attachment of the tentorium to the occipital bone.
The falx cerebelli is a sickle shaped fold of dura that projects between the two hemispheres of cerebellum. It is attached anteriorly to the internal occipital crest, and its posterior free margin contains the occipital sinus. As it descends, the falx cerebelli sometimes divides into two smaller folds, which are lost on the sides of the foramen magnum.
The diaphragma sellae is a small circular horizontal fold, which covers a saddlelike prominence on the upper surface of the sphenoid skull bone located in the middle cranial fossa, dividing it in two halves. The diaphragma sellae also almost completely covers the pituitary gland. It contains a central opening that transmits the funnel-shaped extension of the hypothalamus connecting the pituitary gland to the base of the brain.
Blood Supply
The dura mater receives arterial supply from branches of numerous arteries. The several arteries that supply the dura of the anteria cranial fossa include the meningeal branches of the anterior and posterior ethmoidal arteries and of the internal carotid artery, and a branch from the middle meningeal artery. The middle meningeal artery is commonly damaged in head injuries. The several arteries that supply the middle cranial fossa include middle and accessory meningeal arteries of the internal maxillary artery, a branch from the ascending pharyngeal artery, which enters the skull through the foramen lacerum, and branches from the internal carotid artery, and a recurrent branch from the lacrimal artery. The posterior cranial fossa is supplied by meningeal branches from the occipital artery, one entering the skull through the jugular foramen, and another through the mastoid foramen; the posterior meningeal artery from the vertebral artery; occasional meningeal branches from the ascending pharyngeal artery, entering the skull through the jugular foramen and hypoglossal canal; and a branch from the middle meningeal artery.
The venous drainage of dura mater occurs by ways of meningeal veins that lie in the endosteal layer. The veins of the dura return blood via diploic veins or end in the various sinuses. Many meningeal veins open indirectly to the sinuses through a series of amplullae, called venous lacunae. These venous lacunae are located on either side of the superior sagittal sinus and are often invaginated with arachnoid granulations. The venous lacunae are also present near the transverse and straight sinuses. They connect with the underlying cerebral, diploic, and emissary veins.
Nerve Supply
The dura mater receives its nerve supply from branches of the trigeminal, glossopharyngeal, vagal, second and third spinal, sphenopalatine, optic nerve, and superior cervical ganglia. They supply both unmyelinated and myelinated sensory and autonomic fibers.
Brain Surgery
The dura creates an obstruction to reach the neural tissue of the brain and spinal cord, and therefore must be removed during many different types of brain surgery. The fibrous structure of the dura sometimes prevents visualization of the cortex after craniotomy is performed. Thus, during brain surgery, an opening in the dura must be made, requiring cutting. Cutting of the dura must be made with great care because, as described above may blood vessels and nerves innervate the various dural processes.
First, removal of the bone flap during craniotomy must be performed gently because, in certain areas, the inner skull may be adhered to the outer dura, possibly resulting in tearing of dura which can cause extensive bleeding and damage the underlying cortex.
After careful removal of the bone flap, the surgeon must create a surgical flap in the dura to gain access to underlying cortical structures. This procedure is critical, and an unsteady hand or improper tool can result in damage to the underlying brain structures or blood vessels within the dura, e.g., the arteries which supply the dura with oxygen and nutrients, flow between the inner surface of the skull and the dura, and injury to the arteries in the dura can have serious consequences. The middle meningeal artery, which flows underneath the temporal-parietal skull region, for example, is commonly damaged during dural incision. Such damage can result in an epidural hematoma, and, if not controlled, can lead to increased intra-cranial pressure, cortical damage, and ultimately death of the patient. Moreover, the septal folds define the dural sinuses, and venous drainage is funneled through the invaginations into the jugular venous system. The dura sinuses roughly follow the suture lines of the cranium, and performing operations close to the suture lines may cause damage to the sinuses, causing serious bleeding that is difficult to control.
During incision and retraction of the dura matter, major dural blood vessels and any communicating vessels should be avoided if possible, or cauterized with bipolar cautery if necessary.
Although incision of the dura can be performed in a number of ways, great care is required to avoid damage to the underlying cortex. Generally, the dura is first lifted slightly off the cortex using a dura hook (tissue pick), or alternatively by pushing a threaded needle through the outer dura layer and pulling up. Next, an incision is made by nicking the dura using a scalpel blade, with the cutting edge of the blade held upward.
Alternative methods for cutting the dura exist. For example, in one method, microdissection scissors may be inserted into the nick and used to finish the incision via successive cuts. Alternatively, the dural incision can be completed by lifting the adjacent dura and sliding the flat end of a Woodson neurosurgical elevator (a neurosurgical instrument having at least one flat blade with smooth edges approximately 13 mm long and 3 mm wide) under the dura, using it to gently lift the dura away from the brain while cutting directly on the flat surface of the Woodson with a scalpel knife. In some versions the blade of the instrument has a groove along its length. The scalpel knife is placed on the blade, and the Woodson and knife can be advanced together, with the knife cutting the dura sliding on top of the Woodson and the brain remaining protected.
The microdissection scissors are not ideal because they are not able to make a continuous incision. The Woodson technique for incising the dura is not ideal because it requires using two hands in a sometimes awkward position, or requires an assistant to hold one of the two instruments and coordinate the effort.
There exists a need for improved dural cutting devices that make cutting the dura in brain surgery easier and safer by minimizing the risk of inadvertent injury to an underlying blood vessel of the brain tissue that can be operated efficiently by a surgeon.